1 Field Of The Invention:
The present invention relates to clamping jaws of a machine for joining strips By welding, a heat treatment device for the heat treatment of a weld joining Two successive ends of strip by electromagnetic induction and a joining machine suited to the heat treatment by induction of welds for joining steel strips.
The invention relates in particular to welds for joining steel strips running continuously in a cold rolling installation or an installation for treatment such as surface pickling, continuous annealing, electroplating or dip coating and in particular to welds produced using a laser beam and more particularly to heat treatment preceding and/or immediately following the production of said welds. In the rest of this document, the expression “treatment installation” will be used to refer to rolling installations or to the above mentioned treatment installations.
In order to improve the productivity of steel strip treatment installations, avoiding in particular the transformation of said strips reel by reel, modern high-capacity treatment installations are capable of operating continuously by joining the ends of steel strips supplied successively in the form of a reel at the input to the treatment installation, the tail of a strip at the end of treatment being joined to the head of a new strip inserted in the input of the treatment installation.
During this joining operation, the movement of the two ends of strip to be welded into the treatment installation, i.e. the tail of the strip at the end of treatment and the head of the new strip to be treated, is stopped and the sections downstream of the treatment installation (i.e. the sections situated after said tail of the strip according to the direction of travel of the strip) are fed by a strip accumulation device previously filled during the period of time separating two successive joins. Such a joining operation is well known to a person skilled in the art whose efforts relate to the speed of the joining operation, so as to limit the stoppage time for the end of strip and, consequently, the capacity and cost of the accumulation devices.
The joining operation is undertaken by a welding joining machine or welding machine which comprises, in addition to the welding device itself, two pairs of clamping jaws intended to immobilize the strips during joining by welding, respectively a first pair of clamping jaws intended to immobilize the tail of the strip engaged in a section of the treatment installation situated downstream in the direction of travel of the strip and a second pair of clamping jaws intended to immobilize the head of the new strip introduced upstream of the joining machine. Different welding methods used by different welding devices are known to a person skilled in the art. This involves for example flash butt welding, resistance seam welding, MIG, TIG or laser or laser hybrid welding.
The welding joining machine must be capable of producing a high-quality weld. In fact, the breakage of a poor-quality weld during movement of the strip in the treatment installation, or even the need to re-make a weld deemed incorrect or of poor quality may give rise to serious production losses. The decisive elements in the quality of a weld are essentially:                the metallurgical quality of the welded joint, particularly for steels susceptible to metallurgical alterations of the zone thermally affected by the welding operation,        the geometry of the welded section, which should ideally be free from excessive thickness and/or under-thickness in relation to the section of the strip,        the continuity and compactness of the welded joint.        
The metallurgical quality of the welded joint, or of the weld, depends essentially on the welding method used and the thermal cycle it induces in the zone affected by welding, and the different pre- and post-heating or annealing treatments applied locally in the welding machine itself or immediately downstream of the welding machine.
The geometry of the welded section depends on the welding method and the means of completing the weld produced after welding. For example, flash butt welding produces a bead which has to be planed and flash butt welding machines are generally equipped with a planing unit integrated into the welding machine. Resistance seam welding also produces an excessive thickness due to the overlapping of the sheets to be welded and which usually has to be flattened by roller devices integrated into the welding machine. Laser welding allows fine management of the welded section also associated with a very limited zone affected by heat.
The continuity and compactness of the welded joint depend essentially on the welding parameters used. These welding parameters are mostly electrical parameters which are easily and reliably manageable. However, another parameter is vital in order to guarantee the continuity and compactness of the welded joint. This involves the straightness of the edges to be welded and their relative positioning during welding. The edges to be welded refer to the end of the tail of the strip at the end of treatment opposite the end of the new strip engaged in the treatment installation.
In order to guarantee the quality of the joining weld, it is necessary that the ends of the strips to be welded are perfectly aligned and rectilinear. For this purpose, welding machines generally include said two pairs of clamping jaws intended to immobilize the ends of strips intended to be joined by welding. Generally, said ends of strips are immobilized in the clamping jaws with a slight overhang resulting from their precise and clean cropping by means of shears integrated into the welding machine or, in certain cases, from their cutting by means of a laser beam. Also, centering devices integrated into or attached to the welding machine make it possible to align the ends of strips before their immobilization in the clamping jaws.
The considerable broadening of the ranges of steel grades and thicknesses treated in treatment installations and the ever increasing productivity requirements have led to more and more frequent use of welding machines involving the laser welding method. This laser method in fact makes it possible to extend the range of products able to be welded toward very fine thicknesses. It also makes it possible to limit the zones thermally affected for a very wide variety of steel grades, including special metallurgy steels developed for the automobile industry. However, in view of the small thickness of the laser beam, the relative positioning of the edges to be welded has to be extremely precise, as described in the applicant's application EP 1591190.
Although the laser welding method, owing to the extreme concentration of the welding energy, results in reduced affected zones, numerous steel grades with high characteristics are nonetheless still susceptible to hardening and, correlatively, to fragilization. In order to prevent this fragilization and this hardening, heat treatment of the weld may be necessary. Different types of heat treatment of the weld may be used, either by annealing after welding, the purpose of which is metallurgically to restore acceptable ductility to the weld, or by pre-heating capable of reducing the cooling rate of the weld and thus limiting its hardening.
In particular, annealing of joining welds by means of heat treatment by electromagnetic induction is known to a person skilled in the art. For this purpose, a heat treatment device using electromagnetic induction is placed downstream of the welding machine. Once the joint has been welded, the clamping jaws holding the ends of strips in place are opened and the welded joint is moved in the direction of travel of the strip to the electromagnetic induction heat treatment device, where said welded joint is processed by annealing. The annealing of the joining weld by means of an electromagnetic induction heat treatment device situated downstream of the welding machine unfortunately implies a great extension of the welding time, requiring a reduction in the speed of travel downstream of the accumulation device or greater accumulation capacities. Also, the time elapsing between the completion of the welded joint and the start of annealing by means of the electromagnetic induction heat treatment device is capable of harming the quality of the weld.
Heat treatment by electromagnetic induction is also described in patents for the continuous production of welded metal tubes from steel strips, such as for example in patent JP 08-174254. In this case, an electromagnetic induction heat treatment device follows the movement of a laser welding head in order to anneal the weld immediately after welding by the welding head. However, the implementation of such an electromagnetic induction heat treatment device following the welding head of the welding machine is not applicable to the weld joining of heads and tails of strips by a strip joining welding machine.
In fact, the joining of strips depends on the use of said clamping jaws, while the latter are not present in the case of tube welding. Said clamping jaws are needed for the precise positioning of the ends of the strips to be welded, in order to guarantee the quality of the weld. Also, in order to guarantee the best possible positioning of the two edges to be welded to one another, and particularly in the case of thin strips (for example less than 1 millimeter thick), the overhang of the edges to be welded—i.e. their protuberance with respect to the clamping jaws and thus the separation of said pairs of clamping jaws from each other, or the inter-jaw space—must be as small as possible.
Owing to this minimization of the inter-jaw space, it is difficult to position an electromagnetic induction heat treatment device close to the weld capable of heating the weld without inducing great risks of heating the clamping jaws by circulation of electrical currents induced by the electromagnetic field (i.e. eddy currents) and consequently disturbances in the heating of the weld or without reducing the precision of the positioning of the ends of strips to be welded. In fact, in the case of joining thin strips, the choice of an inter-jaw space making it possible to limit said heating of the clamping jaws, or simply to free a space for the positioning of the induction treatment device, may result in excessive overhang of the edges to be welded, which consequently no longer allows for the correct relative positioning of said edges to be welded. In particular, in the case of joining extremely thin strips, with a thickness for example between 0.1 and 0.4 mm, the ends of strips to be welded are cut by laser, and not using shears. It follows that no space is placed between the lower clamping jaws (i.e. positioned beneath the strip to be joined) so as to allow the passage of shear blades intended to cut said ends. Consequently, it is impossible to position an electromagnetic induction heat treatment device beneath the strip without positioning the clamping jaws far away from the weld so as to free a space for said heat treatment device, with catastrophic consequences for the quality of positioning of the edges to be welded, or without opening and separating the clamping jaws after welding, with consequences for the complete welding and annealing cycle time, and making it impossible, in this case, to carry out pre-heating of the zone to be welded.